Apparently they do have environment suits on board the Enterprise! Who knew? Though they still don't seem to have any decontamination procedure.

Also, it's tragic that they beam four people over to the Defiant, and then the transporter promptly breaks in such a way that Scotty can only beam back exactly three people at a time, not four.

And then there's the titular Tholian web. You spend almost all of this episode wondering what dramatic thing will happen when the slow, ominous construction is finally completed and the Enterprise is completely trapped...

Yeah. Keep wondering.

Transcript

1
Captain's log, Stardate 5693.2. Searching for USS Defiant, missing 3 weeks.
Sulu: Captain, there it is!
Spock: No life signs.
Kirk: Allons-y!
2
{transporter room}
Kirk: This time, we're wearing environment suits, to prevent any chance of space plague.
McCoy: Where's the fun in that?
3
{Defiant}
McCoy: They're dead, Jim. Everybody, Jim. Everybody's dead, Jim. They're all dead, everybody's dead, Jim.
4
Scotty: {over radio} Defiant's vanishing! And oddly the transporter can only bring 3 of ye back at a time.
Chekov: Vait, vhy is it broken? That vas not foreshadowed.
5
Spock: Beam the captain back, Mr Scott.
McCoy: First let's take off these suits with no decontamination procedure whatsoever.
6
{transporter room}
Spock: Defiant is gone, and the captain with it. We can recover him when it reappears.
Scotty: Aye, like Brigadoon!
7
{bridge}
Loskene: {on viewscreen} You are in Tholian space. Leave now.
Spock: We are engaged in a rescue mission.
Loskene: A likely story. We will ensnare you in our web of death!
8
Loskene: {as Tholian ships begin slowly surrounding Enterprise with a energy web} ... This may take a while...
Spock: This has disrupted Defiant's reappearance. The captain is surely dead.
Chekov: Ve vill be too! Aaargh!
Caption: Insane!
9
{Kirk's quarters}
Spock: You have allowed a plague of murderous aggression onto this ship, doctor.
McCoy: You dimwitted Vulcan, you're not fit to fill Jim's shoes!
McCoy: Also, it was your fault!
10
Kirk: {recording, on computer} If you're playing this message, I am dead. I'm sure you two will get along nicely, now I'm no longer the Harry to your Hermione and Ron.
McCoy: I'm sorry, Spock!
Spock: Forget it, Bones.
11
{bridge}
Uhura: I've seen the captain!
Spock: Are you sure?
Uhura: In my quarters, while I was changing.
Spock: That's him. Mr Scott, beam him back from the other universe!
12
{transporter room}
Kirk: Lucky this suit had a few hours of oxygen.
McCoy: What about the Tholian web?
Spock: We magically escaped it for no apparent reason mere seconds before it was completed.
McCoy: Holy anticlimax.